Allen-Bradley 1785 PLC-5 Modules – 1785-L40B/E

Technical Dossier

Product Details And Specifications

Allen-Bradley 1785 PLC-5 Series Technical Notes

The Allen-Bradley 1785 PLC-5 series represents one of the most widely deployed programmable logic controller platforms in global heavy industry. Installed across petrochemical refineries, nuclear power generation facilities, offshore oil platforms, pulp and paper mills, and large-scale chemical processing plants, the PLC-5 family established the architectural baseline for Rockwell Automation's industrial control ecosystem throughout the 1980s, 1990s, and into the 2000s. Its backplane-based modular architecture, deterministic scan-cycle execution, and broad I/O compatibility made it the default choice for process-critical applications requiring high availability and long service life. Installed bases in facilities with 20–30 year asset lifecycles mean that 1785-series hardware remains operationally active in large numbers globally, despite the platform's formal end-of-life designation by Rockwell Automation.

The Evolution of 1785 PLC-5 Architecture

The PLC-5 platform was introduced in the early 1980s as a successor to the PLC-2 and PLC-3 families, offering expanded memory, faster scan times, and a structured backplane I/O chassis system. Early variants such as the 1785-L11B and 1785-L20B established the core instruction set and DH+ communication model. Mid-generation processors including the 1785-L40B and 1785-L60B introduced expanded data table memory and enhanced remote I/O (RIO) capabilities, supporting larger distributed I/O networks across plant floors.

The /E and /F hardware revisions introduced improved EEPROM memory backup, enhanced EMI tolerance, and revised power supply compatibility. The 1785-L80B and 1785-L80E represented the peak of the platform's native capability, supporting up to 3,072 discrete I/O points and 100KB of user memory. Communication evolution within the series progressed from DH+ (Data Highway Plus) to Ethernet/IP via the 1785-ENET adapter, extending the platform's integration capability into modern SCADA and MES environments.

Rockwell Automation announced end-of-life for the PLC-5 platform in 2015, with final production ceasing in 2022. Migration paths recommended by Rockwell include the ControlLogix (1756) and CompactLogix (1769) platforms. However, due to the complexity and cost of migration in legacy process environments, a significant installed base continues to operate on 1785 hardware, creating sustained demand for spare parts, replacement processors, and I/O modules.

1785 PLC-5 Full Catalog & Functionalities (SKU List)

Processor / CPU Modules

• 1785-L11B: PLC-5/11 processor, 8K user memory, DH+ port, 512 discrete I/O max.
• 1785-L20B: PLC-5/20 processor, 16K user memory, DH+ and RIO ports, 1,024 I/O max.
• 1785-L20B/E: PLC-5/20E processor, enhanced EEPROM backup, 16K memory, Ethernet-ready.
• 1785-L30B: PLC-5/30 processor, 32K user memory, dual RIO ports, 2,048 I/O max.
• 1785-L40B: PLC-5/40 processor, 48K user memory, dual DH+ and RIO, 3,072 I/O max.
• 1785-L40B/E: PLC-5/40E processor, 48K memory, EEPROM backup, enhanced EMI shielding, dual RIO scanner ports — the subject of this listing.
• 1785-L40C15: PLC-5/40C15 processor, ControlNet communication port, 48K memory, 3,072 I/O.
• 1785-L60B: PLC-5/60 processor, 64K user memory, high-density I/O support, dual RIO.
• 1785-L60B/E: PLC-5/60E processor, EEPROM backup revision, 64K memory, enhanced diagnostics.
• 1785-L80B: PLC-5/80 processor, 100K user memory, maximum I/O density, dual DH+ and RIO.
• 1785-L80E: PLC-5/80E processor, EEPROM-enhanced, 100K memory, highest-capacity 1785 CPU.

Communication & Network Adapter Modules

• 1785-ENET: Ethernet/IP communication module for PLC-5 backplane, enables TCP/IP integration.
• 1785-KA: DH+ to RS-232 communication adapter, serial bridge for legacy HMI connectivity.
• 1785-KA5: Enhanced DH+ communication adapter, supports multiple baud rates and node addressing.
• 1785-ME16: 16K EEPROM memory module for program backup and transfer across 1785 processors.

Power Supply Modules (Compatible)

• 1771-P4S: 5VDC/1.5A power supply for 1771 I/O chassis, standard companion to 1785 processors.
• 1771-P7: High-capacity power supply, 5VDC/10A, for large 1771 chassis configurations.

Sourcing Hard-to-Find & Obsolete 1785 PLC-5 Parts

All 1785-series units offered by DriveKNMS are subject to pre-sale inspection, functional verification, and documentation review. For processors such as the 1785-L40B/E, this includes battery condition assessment, EEPROM integrity verification, and backplane connector inspection. Units with degraded batteries or corrupted memory are reconditioned or excluded from inventory prior to listing.

Quality Control for the 1785 PLC-5 Range

The 1785 PLC-5 processor family presents specific quality control requirements due to its backplane-coupled architecture and battery-backed SRAM memory design. DriveKNMS applies the following verification protocol to all 1785-series units prior to dispatch:

• Backplane connector inspection: Physical examination of the 60-pin backplane edge connector for pin deformation, oxidation, and contact integrity.
• Battery voltage test: Lithium battery voltage measurement; units below 2.8V are replaced with OEM-equivalent cells prior to shipment.
• EEPROM read/write verification: For /E and /F revision processors, EEPROM module integrity is confirmed via read-back comparison against a known-good reference program.
• DH+ communication test: Each processor is connected to a live DH+ network segment and verified for correct node addressing, token passing, and message response.
• RIO scanner port test: Remote I/O scanner ports are tested against a 1771 I/O chassis to confirm rack addressing, block transfer execution, and fault detection response.
• Power-on self-test (POST) verification: Processor is powered through a full POST cycle; fault codes are logged and cleared prior to shipment.
• Scan time benchmark: A standardized ladder logic program is loaded and scan time is measured against published specifications for the specific processor model.